The need for minerals with a fine size (nanometers) has increased in recent years. With the resultant increase in ultrafine grinding, comminution science has achieved a range of micrometer sizes. For severe milling, high-quality grinding media is necessary, and various state-of-the-art production technologies have been developed.

The dripping of metal oxides derives from the stocking process of nuclear fuel cells. Recently, this technique has been applied in the ceramic and pharmaceutical industries. One of the most important applications, grinding media equipment, has undergone improvements in many of its mechanical properties. In fact, the most important quality of grinding media is wear resistance.

Due to their better resistance to fading when compared to dyes, water-based pigmented inks are gaining interest for recent inkjet technology developments. The size and shape of these particles, together with the degree of dispersion and the tendency to agglomerate, are important parameters for ink manufacturing that can be met through the use of an appropriate grinding media.

Identifying an Ideal Grinding Media

The ideal media for ultrafine grinding features several reproducible characteristics¹:

•  Chemical composition

•  Hardness (related to chemical composition and grain size)

•  High sphericity

•  High roundness

•  Competency (mechanical integrity)

•  Specific gravity, as designed for machine operation/ore breakage requirements

Bulk density, hardness and fracture toughness are the key physical properties of a ceramic bead. The bulk density has a significant influence on the mill power absorption. The wear resistance, hardness and fracture toughness of the ceramic media also influence mill parameters, such as energy efficiency, internal wear and operating costs. Property advantages, reasonable cost and a low mineral surface degradation are the goals of a good ultrafine grinding process.

Synthesizing Grinding Media

A recent study investigated a forming method that uses sol-gel technology to synthesize ceramic microspheres as grinding media via ceramic slurry dripping. A newly developed suspension process for actinide oxides and metal oxides (e.g., Al₂O₃, TiO₂, SiO₂, ZrO₂, HfO₂, CeO₂) was used. The sphericity and surface smoothness of particles produced by these processes are crucial, as these properties are traditionally desirable.

Drip casting is a process that produces alumina beads from an alumina sol by dripping a ceramic suspension through a nozzle plate to form droplets, and then hardening the droplets in a saline solution. This can be achieved via in situ solidification of ceramic slurry by the polymerization of sodium alginate monomers. Sodium alginate is the sodium salt of alginic acid, a polysaccharide composed of mannuronic and guluronic acids (the acids are produced naturally by brown seaweeds). The ceramic particles are maintained in a three-dimensional network. The mechanism of crosslinking in alginate gels can be considered in terms of an “egg box” model involving cooperative bonding of divalent metal ions between aligned polyguluronate ribbons (Braccini I., 1999).

 The gravitational force induces a ceramic suspension to drip into a salt solution (see Figure 1). At this point, the gelation polymer in the slurry turns into spheres, in which the sodium cation is replaced by divalent cation and immediate, irreversible gelation takes place.

With this forming method, it is possible to produce a variety of ceramic microspheres, such as grinding media and catalyst supports. By sintering the drip-cast particles, it is possible to achieve good mechanical strength in the ceramic beads. In order to produce ceramic particles with maximum strength, the particles must contain minimum porosity, and pores must be kept as small as possible. Particles should be spherical, with a smooth surface and monomodal size.

Analyzing Drip Casting Effectiveness

The goal of analyzing dripped alumina spheres is to prove the effectiveness of drip casting when producing ceramic grinding media. Through the modification of the microstructure characteristics, raw materials and ceramic production processes directly affect all the properties of ceramics, including mechanical properties such as compressive resistance, fracture toughness, hardness, and abrasion resistance. Samples of 92% alumina produced through drip casting were analyzed by measuring the specific gravity and sphericity; additional analyses included a wear test, a scanning electron microscope (SEM) image, X-ray diffraction (XRD), and mechanical properties with a crush test.

The XRD analysis shows the dripping spheres’ composition, and it is possible to observe the absence of other chemical elements. In fact, although the drip casting used an excess of sodium (derived from alginate) and calcium (derived from salt solution), the diffractometer analysis lacks any trace of such elements.

The specific gravity of the drip-cast alumina spheres goes up to a value of 3.70 g/cc. This bulk density increase could mean that the drip casting technique increases densities during sintering. High density is a desirable property in ceramic grinding media; in the wear tests, the spheres with an elevated density show more resistance than those with a low density. Another confirmation of this hypothesis has been obtained with internal observation of sphere samples. The ceramic spheres seem full and densely packed. Despite some small, closed porosity, the beads do not exhibit macroscopic defects.

The internal aspect is easy to see after the spheres have been cut. The dripped spheres appear to exhibit a good density, and the roundness of the media is regular, with a high sphericity grade (near the value of unit). The average of the measurements from the entire perimeter is representative of the surface roundness of the beads, which has been measured by an optical profilometer; thus, the sphericity (or aspect) near one has been evaluated for all dripping beads. Table 1 shows the geometrical parameters of spheres formed by drip casting.

The strength of a ceramic sphere can be determined from the proppant crush resistance test described in ISO 13503-2: Measurement of Properties of Proppants Used in Hydraulic Fracturing and Gravel-Packing Operations. In this test, a sample of proppant is first sieved to remove any fines (undersized pellets or fragments that may be present), then placed in a crush cell where a piston is used to apply a confined closure stress of some magnitude (Newton) above the failure point of some fraction of the proppant pellets. The sample is then re-sieved, and the weight percent of the fines generated as a result of pellet failure is reported as percent crush. A comparison the percent crush of two equally sized samples is a method of gauging the relative strength.

Drip Casting Versatility

The good results of the wear test and crush test confirm the hypothesis that the drip casting achievement is a good synthesis method. Further developments in the field of drip casting are being made. Due to the ceramic synthesis technology, finer particle sizes can be produced, in sub-micrometer order (e.g., 0.01-0.10 µm).

In addition, the drip casting technique can be applied to many different substances, offering a new manufacturing technique for many applications. The simplicity of this technology allows for an efficient manufacturing process and offers the possibility to modify the initial setting of a project to suit specific purposes. The knowledge behind drop formation physics helps laboratory technicians anticipate the shape and the route of the drop. Finally, drip casting provides technicians the opportunity to be creative and come up with as many different types of spheres as possible.

This technology’s versatility has enabled the study of many different products, each beginning with the idea that a “universal” type of grinding medium does not exist. Every single formulation has its own properties and method of application. 

Reference

 1. Lichter, J. and Davey, G., “Selection and Sizing of Ultrafine and Stirred Grinding Mills,” SME Mineral Processing Plant Design Symposium, Vancouver, Canada, 2002.